Sequential chemical vapor deposition

ABSTRACT

The present invention provides for sequential chemical vapor deposition by employing a reactor operated at low pressure, a pump to remove excess reactants, and a line to introduce gas into the reactor through a valve. A first reactant forms a monolayer on the part to be coated, while the second reactant passes through a radical generator which partially decomposes or activates the second reactant into a gaseous radical before it impinges on the monolayer. This second reactant does not necessarily form a monolayer but is available to react with the monolayer. A pump removes the excess second reactant and reaction products completing the process cycle. The process cycle can be repeated to grow the desired thickness of film.

This application is a continuation-in-part of U.S. application Ser. No.08/699,002, filed on Aug. 16, 1996, which is incorporated herein byreference. The present invention relates to methods and apparatusessuited to the low temperature deposition of solid thin films of one ormore elements by the technique of sequentially exposing the object beingcoated with chemically reactive gaseous species. It also describes anumber of applications of films produced by such processes.

BACKGROUND OF THE INVENTION CVD Reactor Technology

Chemical vapor deposition (CVD) reactors have been used for decades todeposit solid thin films and typical applications are coating tools,manufacture of integrated circuits, and coating jewelry. A. Sherman,Chemical Vapor Deposition for Microelectronics, Noyes Publications, NewJersey, 1987. Up to the 1960's many CVD reactors operated by exposing aheated object or substrate to the steady flow of a chemically reactivegas or gases at either atmospheric or reduced pressures. Since, ingeneral, it has been desired to deposit films at as high a rate aspossible as well as at as low a temperature as practical, the gases usedto produce the film are extremely reactive (e.g., silane plus oxygen todeposit silicon dioxide). Then if the gases are allowed to mix for toolong a time period before impinging the substrate, gas phase reactionscan occur, and in extreme cases there can be gas phase nucleation andparticles formed rather than deposition of continuous films. At the sametime, the high rate of deposition and the reactive gases used makes itvery difficult to coat large area substrates uniformly. This results invery complex and expensive commercial CVD reactors. A furthercomplication with this method is that in some cases the films depositeddo not conformally coat non-uniform surfaces. This can be particularlydeleterious in the manufacture of integrated circuits.

In the 1960's it was realized that we could lower the temperaturerequired for thin film deposition at acceptable rates by creating a lowpressure glow discharge in the reactive gas mixture. The glow dischargeproduces many high energy electrons that partially decompose thereactive gases, and these gas fragments (radicals) are very reactivewhen they impinge on a surface even at moderate temperatures. Althoughusing a glow discharge allows lower temperature operation, commercialreactors are very complex and expensive, since uniform deposition overlarge area substrates is even more difficult due to the inherentnonuniformity of glow discharges and due to the added expense of complexhigh frequency power supplies. Also, this technique can often lead todegradation of the film conformality, due to the highly reactive natureof the radicals.

In the 1970's atomic layer epitaxy (ALE) was developed in Finland by T.Suntola and J. Anston. U.S. Pat. No. 4,058,430 describes how they grewsolid thin films on heated objects. This process involves exposing theheated surface to a first evaporated gaseous element, allowing amonolayer of the element to form on the surface, and then removing theexcess by evacuating the chamber with a vacuum pump. When a layer ofatoms or molecules one atom or molecule thick cover all or part of asurface; it is referred to as a monolayer. Next, a second evaporatedgaseous element is introduced into the reactor chamber. The first andsecond elements combine to produce a solid thin compound monolayer film.Once the compound film has been formed, the excess of the second elementis removed by again evacuating the chamber with the vacuum pump. Thedesired film thickness is built up by repeating the process cycle many(e.g., thousands) times.

An improvement to this technique was described in a later patent issuingin 1983 to T. Suntola, A. Paakala and S. Lindfors, U.S. Pat. No.4,389,973. Their films were grown from gaseous compounds rather thanevaporated elements so the process more closely resembles CVD. This wasrecognized to be especially advantageous when one component of thedesired film is a metal with low vapor pressure, since evaporation ofmetals is a difficult process to control. With this approach, films weredeposited by flow reactors similar to a conventional CVD reactor, wherethe excess of each gas is removed by flowing a purge gas through thereactor between each exposure cycle. This approach was limited to only afew films, depending on the available gaseous precursors, and all ofthese films were not as contamination free as desired. We will refer tothis process as sequential chemical vapor deposition.

An alternative approach to operating a sequential chemical vapordeposition reactor would be to operate a non-flow vacuum system wherethe excess gaseous compound of each sequence is removed by vacuum pumpsin a manner similar to the original Suntola 1977 process. H. Kumagai, K.Toyoda, M. Matsumoto and M. Obara, Comparative Study of Al ₂ O ₃ OpticalCrystalline Thin Films Grown by Vapor Combinations of Al(CH ₃)₃ /N ₂ Oand Al(CH ₃)₃ /H ₂ O ₂, Jpn. Appl. Phys. Vol. 32, 6137 (1993).

An early application of sequential chemical vapor deposition was fordeposition of polycrystalline ZnS thin films for use in electrochromicflat panel displays. M. Leskela, Atomic Layer Epitaxy in the Growth ofPolycrystalline and Amorphous Films, Acta Polytechnica Scandinvica,Chapter 195, 1990. Additional studies have shown that other commerciallyimportant solid films of different compounds, amorphous andpolycrystalline, can be deposited by this technique on large area glasssubstrates. Among these other films are sulfides (strontium sulfide,calcium sulfide), transition metal nitrides (titanium nitride) andoxides (indium tin oxide, titanium dioxide). Elsewhere, this techniquehas been developed as a means of depositing epitaxial layers of groupIII-V (gallium indium phosphide) and group II-VI (zinc selenide)semiconductors, as an alternative to the much more expensive molecularbeam epitaxy process.

To applicant's knowledge the only literature discussing sequentialchemical vapor deposition of elemental films are those that depositelemental semiconductors in group IVB such as silicon and germanium. Onesuch study, S. M. Bedair, Atomic Layer Epitaxy Deposition Process, J.Vac. Sci. Technol. B 12 (1), 179 (1994) describes the deposition ofsilicon from dichlorosilane and atomic hydrogen produced by a hottungsten filament. By operating the process at 650° C. deposition ofepitaxial films are described. Deposition of diamond, tin and leadfilms, in addition to silicon and germanium by an extraction/exchangemethod in conjunction with a sequential processing scheme similar tosequential chemical vapor deposition has also been reported M. Yoder,U.S. Pat. No. 5,225,366. Also although some of the studies reported haveexplored processes that may be useful at moderate temperatures, mostrequire undesirably high substrate temperatures (300-600° C.) to achievethe desired sequential chemical vapor deposition growth of high qualityfilms.

Conformal Films Deposited at Low Temperatures for Integrated CircuitManufacture

A continuing problem in the commercial manufacture of integratedcircuits is the achievement of conformal deposition of dielectric (e.g.,silicon dioxide, silicon nitride) or conducting (e.g., aluminum,titanium nitride) thin solid films over large area wafers (e.g., 12inches in diameter). A film is conformal when it exactly replicates theshape of the surface it is being deposited on.

In one paper by D. J. Ehrlich and J. Melngailis, Fast Room-TemperatureGrowth of SiO ₂ Films by Molecular-layer Dosing, Appl. Phys. Lett. 58,2675 (1991) an attempt was reported of layer by layer deposition ofsilicon dioxide from silicon tetrachloride and water. Although the filmsappear to be very conformal, there is no discussion of film quality ordensity, and it is likely that these films are porous making themunsuitable for thin film applications. In support of this conclusion, wecan refer to a study by J. F. Fan, K. Sugioka and K. Toyoda,Low-Temperature Growth of Thin Films of Al ₂ O ₃ with Trimethylaluminumand Hydrogen Peroxide, Mat. Res. Soc. Symp. Proc. 222, 327 (1991). Here,aluminum oxide deposited at 150° C. was compared to deposition at roomtemperature. In this case, the room temperature films thickness reducedfrom 2270 Å to 1200 Å upon annealing at 150° C. for 15 minutesconfirming the high porosity of the film deposited at room temperature.Another attempt to deposit silicon dioxide by sequential chemical vapordeposition used silane and oxygen by M. Nakano, H. Sakaue, H. Kawamoto,A. Nagata and M. Hirose, Digital Chemical Vapor Deposition of SiO ₂,Appl. Phys. Lett. 57, 1096 (1990). Although these films, deposited at300° C., appeared to be of better quality, they were not perfectlyconformal, and could only fill holes of an aspect ratio up to 3:1.Modern integrated circuit technology requires the ability to coat holesand trenches with aspect ratios well in excess of 3:1.

Another technologically important thin solid film that needs to bedeposited with high purity and at low temperature, conformally overlarge area wafers, is the multilayer film of titanium and/or titaniumsilicide plus titanium nitride. Here, the need is for a thin titaniumand/or titanium silicide layer to be deposited on a silicon contact (100Å) followed by a layer of titanium nitride (3-400 Å). In a recent paperby K. Hiramatsu, H. Ohnishi, T. Takahama and K. Yamanishi, Formation ofTIN Films with Low Cl Concentration by Pulsed Plasma Chemical VaporDeposition, J. Vac. Sci. Techn. A14 (3), 1037 (1996), the authors showthat an alternating sequence process can deposit titanium nitride filmsat 200° C. from titanium tetrachloride and hydrogen and nitrogen.However, the chlorine content of the films was 1%, and no attempt wasmade to deposit pure titanium metal or titanium silicide. Also, thereactor used was very similar to the conventional expensive plasmaenhanced CVD reactor.

Finally, sputtered aluminum films have been widely used to fabricateintegrated circuits for many years. Unfortunately, sputtering is a lineof sight deposition technique, so the films tend to be non-conformal.This has become more of a problem, in recent years, as denser circuitdesigns have resulted in holes of high aspect ratio that need to befilled. For this reason, many attempts have been made to find a suitablechemical vapor deposition process that would be highly conformal, andseveral processes have been successfully demonstrated by R. A. Levy andM. L. Green, Low Pressure Chemical Vapor Deposition of Tungsten andAluminum for VLSI Applications, J. Electrochem. Soc. Vol. 134, 37C(1987). Although conformal thin films of aluminum can be deposited byCVD, these films are still not acceptable for use in circuits, becausealuminum is susceptible to electromigration and it is preferred to addseveral percent of copper to these films to avoid this problem. All butone attempt to carry out the CVD process with copper precursors added tothe aluminum precursors have been unsuccessful. See E. Kondoh, Y.Kawano, N. Takeyasu and T. Ohta, Interconnection Formation by DopingChemical-Vapor-Deposition Aluminum with Copper Simultaneously: Al—CuCVD, J. Electrochem. Soc. Vol. 141, 3494 (1994). The problem is thatalthough there are CVD processes for the deposition of copper, theprecursors used interact with the aluminum precursors in the gas phasepreventing the simultaneous deposition of aluminum and copper.

Composite Fabrication

Many schemes have been developed to fabricate composite materials,because of the unusual strength of such materials. One approach to thefabrication of such materials is to prepare a cloth preform (e.g. fromthreads prepared from carbon fibers), and then expose this preform to ahydrocarbon gas at high temperatures. The hydrocarbon then pyrolyseswith carbon depositing on the carbon preform. Unfortunately, thisprocess is not very conformal, so that the outer pores of the preformare sealed before the interior can be coated, and the process has to bestopped prematurely. The preform then has to be machined to remove theouter layer, and further exposure is needed. This is a slow and veryexpensive process which is referred to in the literature as ChemicalVapor Infiltration (CVI); see e.g., Proceedings of the TwelfthInternational Symposium on Chemical Vapor Deposition 1993, eds. K F.Jensen and G. W. Cullen, Proceedings Vol. 93-2, The ElectrochemicalSociety, Pennington, N.J.

Coating Aluminum with Aluminum Oxide

As is well known, coating aluminum with a thin layer of oxide is anexcellent way to protect this material from corrosion by the elements.The traditional way of doing this is to anodize the aluminum with a wetelectrochemical process (Corrosion of Aluminum and Aluminum Alloys, Vol.13 of Metals Handbook, ASM, Metals Park, Ohio, 1989). Pinholes and otherdefects in the anodized layer are the source of local failure of thecorrosion protection of the anodized layer. Such pinholes occur becausethe wet anodization process relies on the underlying aluminum as thesource of the aluminum in the aluminum oxide coating, and the underlyingaluminum can have many impurities and defects. A preferred approachwould be to deposit the desired aluminum oxide from an external source.Although using a CVD process to carry this out is a possible choice,this has not been explored because the traditional CVD process operatesat 1000° C., and this far exceeds the melting point of the underlyingaluminum.

Low Temperature Brazing

In the manufacture of high temperature, high density ceramics, there isgreat difficulty in fabricating unusual shapes to high accuracy. Mostoften the ceramic is formed in the “green” state, machined while stillsoft, and then fired at high temperature. After firing, the resultinghigh density ceramic part may require additional machining, for example,with diamond grinding wheels, to achieve the desired dimensionalaccuracy. In some cases, the part shape makes this additional machiningdifficult and expensive, and in some instances there may be no known wayto reach the surface that needs to be ground. High temperature brazingof ceramic parts is an alternate technology for joining odd shapes ofaccurately finished ceramics. In some instances the braze metal may notbe compatible with the desired application. Also the high temperaturepreferred for metal brazing makes it difficult to join parts ofdifferent thermal expansion coefficients. For example, it is notpossible to braze aluminum to alumina ceramic, because the traditionalbrazing temperature would be far higher than the melting point of thealuminum.

SUMMARY OF THE INVENTION

In one embodiment, the present invention provides a reactor operated atlow pressure, a pump to remove excess reactants, and a line to introducegas into the reactor through a valve. In this embodiment, a firstreactant forms a monolayer on the part to be coated, while the secondreactant passes through a radical generator which partially decomposesor activates the second reactant into a gaseous radical before itimpinges on the monolayer. This second reactant does not necessarilyform a monolayer but is available to react with the monolayer. A pumpremoves the excess second reactant and reaction products completing theprocess cycle. The process cycle can be repeated to grow the desiredthickness of film.

Because the film can be deposited one monolayer at a time, the filmforming on the part tends to be conformal and have uniform thickness.The present invention may use inexpensive reactors that can coat manyparts simultaneously reducing costs. For the formation of athree-element film, an additional step introduces a third reactant inthe process cycle. A stable compound film of any number of elements canbe formed by growing the monolayers of the elements with gaseousprecursors that contain the elements. Such precursors can be halides ororganometallic compounds.

It is an object of the invention to facilitate the growth of thin filmsof any element by using a radical generator to make available highlyreactive gases (radicals).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of a sequential CVD reactor, suitable forthe deposition of films that are not electrically conducting,constructed in accordance with one embodiment of the present invention.

FIG. 2 illustrates a process cycle for the sequential CVD process.

FIG. 3 is a schematic drawing of a sequential CVD reactor, suitable forthe deposition of any film, conducting or non-conducting, constructed inaccordance with an embodiment of the present invention.

FIG. 4 illustrates an alternative process cycle for the sequential CVDprocess.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a cross-section view of a reactor vessel 2 made of anon-conducting dielectric ceramic (e.g. a quartz cylinder) which issuitable for the deposition of a non-electrically conducting film on anon-electrically conducting part. The reactor vessel 2 forms a chamberclosed at one end by a flange 8, through which gases are introduced, andclosed at the other end by a flange 4 which connects to a vacuum pump 38through a pneumatically operated solenoid gate valve 36. Each flange hasan O-ring seal 6 to allow vacuum operation. The part 12 is placed in thereactor vessel 2 on a non-electrically conducting part holder 10. Avacuum gage 26 monitors the chamber pressure during operation. A firstreactant 28 is introduced as a gas into the chamber by evaporating aliquid or solid contained in bottle 30 by temperature controller 32 toprovide adequate vapor pressure for delivery into the chamber. In manysituations, the temperature controller 32 will provide heat to the firstreactant in the bottle 30. However, in others the controller may coolthe first reactant 28 in the bottle 30.

The first reactant 28 will be a compound having the elements of themonolayer to be formed on the part 12 such as the first reactants listedbelow in Examples 1-7. The first reactant 28 is introduced into thereactor vessel 2 through solenoid operated pneumatic valve 20 by amanifold 18. FIG. 1 illustrates a system with two bottles 30 and 31,each containing a first reactant 28 and 29, however, the type of film tobe formed will determine the number of reactants and bottles. Forexample, if a ternary film is desired, the system will include threebottles and three valves. A conventional digital microcontroller 40sequences the opening and closing of the valves 20 and 22 to deliver thefirst reactants to the chamber at the appropriate times as illustratedin FIG. 2.

Referring to FIG. 1, during a typical operation, a monolayer of thefirst reactant is deposited on the part 12 to be coated by exposure tothe first reactant 28 in vapor phase from the bottle 30. This monolayeris reacted by exposing it to a flux of radicals generated by the actionof a solenoidal coil 14, excited by a RF power supply 16, on moleculesintroduced from a gas bottle 34. The RF power supply 16 can becontrolled by the microcontroller circuit 40.

FIG. 2 illustrates a process cycle for forming thin films with reactorvessel shown in FIG. 1. Initially, the vacuum pump 38 evacuates thechamber of the reactor vessel 2. The exhaust gate valve 36 then closesand a valve 20 opens for a short period of time to deliver the firstreactant 28 to the reactor vessel 2 in a sufficient amount to form amonolayer of molecules on the part 12 to be coated. After the monolayeris formed, the reactor vessel 2 is again evacuated by the vacuum pump 38to remove excess first reactant. Next, a second reactant from bottle 34is delivered into the reactor vessel 2 for a short period of time whilea solenoidal coil 14 is excited by the RF power supply 16 generatingradicals. This step is carried out for a sufficient period of time tofully react the radicals with the first reactant monolayer. Finally, thereactor vessel 2 is evacuated again by the vacuum pump 38 ending thefirst cycle. The process cycle can then repeat to form the desiredthickness of the film.

If the film to be deposited is electrically conducting, reactor vessel 2will be coated by a conducting film which eventually shields out theexciting electric field provided by the solenoidal coil 14. To avoidunnecessary reactor vessel cleaning, in another embodiment, the presentinvention provides the reactor vessel 3 as shown in FIG. 3. The exhaustflange 4 provides access to the interior of the reactor vessel 3. Theflow of second reactant 42 is generated in a radical generator 44 whichis attached to the wall of the reactor vessel 3. As before the firstreactant 28 is provided from the bottle 30 and introduced to the reactorvessel 3 through the valve 20 and the manifold 18. In this embodiment,the part holder 10 can be either a metal or a ceramic. Again themicrocontroller 40 controls all valves and the radical generator 44.

The radical generator 44, suitable for use with the reactor vessel 3,shown in FIG. 3, can take on many well known arrangements. Onearrangement is to use a miniaturized version of the quartz tube 2 and RFcoil 14 described in FIG. 1. In this arrangement, the only modificationis to provide an end plate with a small hole in it, so that the radicalscan flow rapidly into the reactor vessel 3 through such a nozzle. Oneillustration of a suitable end plate with a hole in it serving as anozzle is shown in FIG. 1, as a stainless steel anode, in a paper by A.Sherman, In situ Removal of Native Oxide from Silicon Wafers, J. Vac,Sci. Technol. Vol. B8 (4), 656 (July/August 1990) which paper isincorporated by reference here in its entirety. This paper alsodescribes generating hydrogen radicals using a hollow cathode DCdischarge chamber. Other alternatives are reviewed for hydrogen radicalgeneration in a recent paper by V. M. Beramudez, Simple, EfficientTechnique for Exposing Surfaces to Hydrogen Atoms, J. Vac. Sci. Technol.Vol. A14, 2671 (1996). Similar techniques can be also used to generateany of the radicals that might be needed to form the elemental filmsdescribed herein.

Concerns about the uniformity of distribution of radicals should notcontrol the type of radical generator 44 to be employed. As long assufficient radicals are generated to react the first reactant, anyexcess radicals play no role in the film formation. More importantconsiderations relate to avoiding the introduction of contaminants, thecost of the radical generator, and simplicity of its operation. Also,the reaction between any one of the first reactants adsorbed on the partsurface and the radical flux to the part, should be rapid andindependent of surface temperature. Therefore, it should be possible tocarry out these thin film depositions at lower temperatures than inconventional sequential chemical vapor deposition processes which aretypically carried out at 300-600° C.

One of the difficulties in the commercial application of traditionalsequential chemical vapor deposition processes, is that they depositfilms slowly. For very thin films (e.g. 100 Å) this is of littleconcern. However, if thicker films are required (e.g., 1 μm or 10,000Å), then the commercial viability of some applications may be inquestion.

In the present process, by virtue of the use of remotely generated, veryreactive, radicals (e.g. oxygen atoms, hydrogen atoms, nitrogen atoms,etc.) we are able to operate the process at room temperature. This factgives rise to two features of this process that can lead to higherthroughput from the reactor used.

When the first reactant is exposed to the substrate at room temperature,it is possible for more than one monolayer to remain behind after thereactor is evacuated with a vacuum pump. In fact, if the substratetemperature is lowered enough we would find the precursor condensing toa liquid film on the substrate surface—obviously not the way to operatethe present process. Then when the substrate, with multiple monolayersremaining on its surface, is exposed to the second reactant (radical)more than one monolayer of product film can be grown in each cycle. Ourexperimental data has verified that 3 Å of Al₂O₃ grows per cycle fromTMA and oxygen atoms at room temperature. All other studies of Al₂O₃formed in thermal (e.g. high temperature) sequential CVD showsdeposition rates of less than 1 Å/cycle.

Second, if we do not have to fully evacuate the reactor chamber aftereach precursor exposure in our process, we could shorten the time foreach cycle. In the flow type reactor described by Suntola in U.S. Pat.No. 4,389,973, he used an inert gas to purge each reactant after eachexposure of the substrate. Typically nitrogen gas was used as the purgegas. In our case, the second reactant is created by striking a glowdischarge in an otherwise inert gas (e.g. O₂→O). Therefore, there is noneed to use a separate inert gas to purge the first reactant. We cansimply use the second gas with the discharge turned off. Again, it isnot necessary to purge the second reactant, because it goes away when weextinguish the glow discharge. By eliminating the separate purge gas, wecan shorten and simplify the deposition cycle. This will enable ashortening of the cycle time.

It should be recognized, however, that there are some instances whereusing a purge gas to separate the two reactants in a sequential CVDreactor may not be the most desirable way to operate the system. Whensubstrates are being coated that have features with high aspect ratioholes or trenches it will, in general, be more effective to use thevacuum pump out style described earlier. This will be the case, becauseit would be harder for a given reactant to diffuse through an inert gasdown to the bottom of a hole when the hole is filled with inert gas. Forthose applications where high aspect ratio holes do not have to becoated (e.g., large area flat panel displays), then the inert gas purgewould be suitable. In that case, using the gas in which a glow dischargeis created as the inert gas (with glow discharge off) for a purgeoperation should enhance throughput.

Finally, when very thin films of dielectric materials (e.g., Al₂O₃,TiO₂, Si₃N₄) are deposited by a sequential CVD process, the surface mayhave a substantial degree of roughness in spite of the layer by layermethod of deposition. Apparently, this phenomenon is caused by somepoorly understood agglomeration process as the film is growing. Onetechnique that can be used to avoid this surface roughening would be togrow many thin layers where two similar materials alternate. Forexample, if we want a 100 Å film we could grow, alternately, 10 Å layersof Al₂O₃ and 10 Å layers of Si₃N₄ and do it 5 times. This should producea dielectric layer with a dielectric constant of about 7-8, which is agood diffusion barrier and has good electrical breakdown strength, andwhich is also very flat. By using the new method described earlier, wecan deposit such a flat multi-layer film at lower temperatures than werepossible before.

Example 1

Deposition of thin films of silicon dioxide can be carried out with asilicon precursor, such as dichlorosilane which can be reduced toelemental silicon by a flux of hydrogen atoms. S. M. Bedair, AtomicLayer Epitaxy Deposition Process, J. Vac. Sci. Technol. B 12 (1), 179(1994). It should also be possible to deposit elemental silicon fromother precursors (e.g., silane, tetramethylsilane) and atomic hydrogen.The resulting silicon can then be converted to silicon dioxide byexposure to oxygen. In this way a silicon dioxide film can be grownmonolayer by monolayer. Another way to grow this film would be to use asilicon precursor that already contains oxygen. For example, one coulduse tetraethoxysilane and reduce it with oxygen atoms.

Example 2

In one embodiment, the present invention provides a process for coatinga part with an elemental metal film. For brevity sake, we will limit thediscussion to a titanium metal film. In this example, the first reactantcould be titanium tetrachloride which could be introduced into thereactor at a low pressure so that a monolayer adsorbs to the surface ofthe part. Next, any excess titanium tetrachloride in the reactor chamberis pumped out. In order to form pure titanium films, we could thenexpose the surface to low pressure hydrogen in atomic form. The hydrogenatoms will react with the chlorine in the titanium tetrachloridemonolayer to form HCl. The HCl vapor can then be exhausted by a vacuumpump, and a monolayer of titanium will be left behind. The thickness ofthe titanium metal film is determined simply by the number of processcycles carried out. By this process it is possible to grow a film of anyelement that is solid at room temperature.

Deposition of thin titanium plus titanium nitride compound films couldbe derived from titanium tetrachloride and hydrogen atoms to yield thepure titanium, followed by exposure to nitrogen atoms to form thenitride. Alternately, we could expose titanium tetrachloride to NHradicals to produce titanium nitride films directly. Again, if we use aprecursor that contains both titanium and nitrogen atoms, e.g.,tetrakis(diethylamino)titanium or tetrakis(dimethylamino)titanium, wecould reduce a monolayer of either of these species with hydrogen atomsor HN radicals to form titanium nitride.

Example 3

The present invention provides for growing a film with three or moreelements such as an oxynitrides by sequentially growing an oxide andthen growing a nitride. In fact, there would be no difficulty in growingternary compounds such as tantalum/silicon/nitrogen which is a gooddiffusion barrier film for advanced integrated circuits.

Various binary and ternary silicides can be formed by depositing one, ormore, metallic or semiconductor elements and nitriding the layer withnitrogen atoms. For example, we could deposit a monolayer of puresilicon, and then a monolayer of pure titanium. If the resultingmonolayer of titanium silicide were then nitrided with a flux ofnitrogen atoms, we could have a titanium/silicon/nitrogen ternarycompound. Also, the stoichiometry of the compound film could be changedsimply by changing the number of cycles used for any of the elements.For example, titanium disilicide (TiSi₂) could be formed from twosilicon cycles for each titanium cycle.

Example 4

Deposition of aluminum films doped with copper and silicon could beformed from triisobutylaluminum, copper(II) acetylacetonate[Cu(acac)₂],and tetramethylsilane each reduced in turn by hydrogen atoms. Thepercentages of copper and/or silicon dopants could be adjusted bycontrolling how many layers of each element are deposited. For example,a two percent doping level of copper is achieved by depositing one layerof copper for every 50 layers of aluminum.

Example 5

If we take full advantage of the ability of the sequential CVD processto conformally coat parts that are very porous, we could fabricate anumber of important composite materials. For example, we could grow acarbon layer from methane and hydrogen atoms. This layer could then beconverted to a silicon carbide by growing a silicon layer as describedin Example 1. This silicon carbide coating could be used to coat acarbon fiber preform until a solid silicon carbide body is formedreinforced with carbon fibers. The carbon fibers would give the partgreat strength, and the silicon carbide would allow it to be used athigh temperatures in air. Ceramic composites using alumina whiskerscould be formed by growing aluminum oxide on a preform made from suchfibers. Metallic composites could be also prepared using metallic fiberpreforms and a sequential CVD to grow metal on the preform.

Example 6

We now know that good quality aluminum oxide thin films can be grown atmoderate temperatures by H. Kumagai, K. Toyoda, M. Matsumoto and M.Obara, Comparative Study of Al ₂ O ₃ Optical Crystalline Thin FilmsGrown by Vapor Combinations of Al(CH ₃)₃ /N ₂ O and Al(CH ₃)₃ /H ₂ O ₂,Jpn. J. Appl. Phys. 32 6137 (1993) by sequential CVD. It is, therefore,possible to coat anodized aluminum parts with this highly conformallayer. The earlier CVD processes could not be used because they had tobe operated at temperatures higher than the melting point of aluminum.One approach would be to use known methods of sequential CVD to coataluminum. An alternative approach would be to take advantage of theprocess described in the present invention, where we can form monolayersof pure aluminum and then oxidize these layers with oxygen atoms. Forexample, we could reduce trimethylaluminum with hydrogen atoms to formthe aluminum layer. This layer will readily oxidize when exposed tooxygen. If the aluminum were initially anodized, the sequential chemicalvapor deposition film will fill in any defects or pinholes.

Example 7

Joining two pieces of ceramic at low temperature with a pure ceramicmaterial, is a process that has some unique advantages. For example, thetemperature tolerance of the joined parts will be as high as theoriginal ceramic parts. Also, no new material is added to the structure,so the resulting joined part is of high purity, and just as chemicallyinert as the original ceramics. Such a process does not exist today. Forexample, two pieces of aluminum oxide could be joined by growingaluminum oxide, as described in Example 6, on the two adjacent parts.

Example 8

The capacitance of a capacitor is directly proportional to thedielectric constant of the dielectric material between the capacitorplates. It is also inversely proportional to the dielectric thickness.When it is desired to increase the capacitance in an integrated circuit,it is traditional to reduce the thickness of the preferred dielectricthermal SiO₂. In modern advanced circuits, the practical limit to SiO₂thickness has been reached (˜30 Å). Attempts to grow uniform pinholefree SiO₂ films thinner than this has proven difficult. An alternativewould be to deposit a dielectric with a higher dielectric constant, andthis would allow a more practical dielectric thickness. For example, ifwe deposit a thin film of Ta₂O₅, with a dielectric constant of 25 (6×that of silicon dioxide), then the 30 Å film could be 180 Å thick. Notonly is this a film thickness that can be reliably deposited, furtherimprovements in capacitance can be achieved by reducing the thickness ofthe Ta₂O₅ further.

Unfortunately, very thin layers of Ta₂O₅ deposited on silicon bytraditional CVD high temperature techniques, result in dielectrics witha dielectric constant of much less than 25. This is because as theprocess is begun, the first thing that happens is that the siliconoxidizes and we end up with a composite layer of Ta₂O₅ and SiO₂. Themuch lower dielectric constant of the SiO₂ lowers the overall value ofthe dielectric constant of the composite film.

In the current process, we can deposit Ta₂O₅ at low temperatures, ifdesired, and thereby minimize any oxidation of the underlying silicon.If, regardless of the low temperatures used, we find that there is stillsome silicon oxidation, we can deposit one or several monolayers of someoxygen barrier material (e.g. TiN, TaN, etc.) or sacrificial material(e.g. Ta) on the silicon before proceeding to the Ta₂O₅ deposition usingoxygen atom radicals.

Example 9

In recent years there has been a tendency to replace the aluminumconductors, in an integrated circuit, with copper conductors. Since itis very difficult to plasma etch copper in the same way that aluminum isetched, most manufacturers have gone to a “Damascene” or inlaidapproach. The traditional technique would be to deposit a layer ofcopper, etch holes in this copper layer, and then fill these holes witha suitable dielectric. Instead, we deposit a layer of dielectricmaterial, etch holes in it, and then coat the entire surface with alayer of copper. This copper fills all the holes previously etched. Thenthe excess copper on the wafer surface is removed with a chemicalmechanical polishing step. Of the several ways that copper can bedeposited, the preferred appears to be electroless plating.Unfortunately, copper cannot be electroplated onto insulator surfaces,so a copper “seed” layer is deposited by CVD. If the conformal coverageof this “seed” layer is good, then the full copper layer can be reliablycoated.

Recent efforts to deposit pure copper thin films by CVD have requiredthe use of complex and expensive copper organometallic compounds. Thisapproach has been found to be preferred because all available copperhalogen compounds, which are inexpensive, are high temperature meltingpoint solids, and they are difficult to vaporize in a controlled fashionfor introduction into the CVD reactor chamber.

With the flexibility of the present process described earlier, we canuse an inexpensive copper-oxygen organometallic compound (e.g., CopperII 2,4-pentanedionate C₁₀H₁₄O₄Cu which is stable, has a vapor pressureof 10 mtorr at 100° C., and is inexpensive) and reduce it to CuO₂ withexposure to oxygen atoms. Then, in a second step the monolayer of CuO₂could be reduced to elemental copper by exposure to hydrogen atoms.Repeating this process for many cycles could produce pure copper thinfilms of any desired thickness. At the same time, if a diffusion barrierlayer is needed between the copper and the underlying Si and SiO₂, suchas TiN, then both layers could be deposited in the same systemsequentially. This could simplify the manufacturing processconsiderably.

Example 10

When depositing a monolayer of some element or compound into a very highaspect ratio blind hole (e.g., 10:1), we first evacuate all gaseousspecies from the hole. Next we expose the hole to precursor moleculesthat adsorb onto the hole surface as well as fill the hole volume. Thenthe precursor molecules occupying the interior hole volume are removedby pumping with a vacuum pump. The next step in the process is to exposethe adsorbed monolayer to a radical flux which then converts it into thedesired monolayer of solid molecular species.

In those cases of extremely high aspect ratio blind holes, anotherphenomena has to be recognized. When the radical flux diffuses into theevacuated volume of the hole, surface reactions release reactionproducts. For example, when adsorbed TMA molecules are attacked byoxygen atoms a monolayer of Al₂O₃ is formed and reaction products suchas H₂O, CO₂, and CO are formed. If the hole is very long and narrow,then it is possible that these reaction product molecules could impedethe diffusion of radicals into the bottom of the blind hole, unless theexposure to radicals was maintained for an impracticably long time.

The solution to this practical difficulty is to expose the very longblind holes to the radical flux in a cyclical fashion, as illustrated inFIG. 4. In other words, after a short exposure of the precursormonolayer to the radicals, evacuate the chamber with a vacuum pump. Thiswill have the effect of removing any gaseous reaction products thatmight tend to prevent radical diffusion into the hole. Then a secondexposure to the radical flux is performed. If preferred, this processcan be repeated many times to achieve the preferred reactions at the endof the very long and narrow blind hole.

Example 11

When depositing metallic films, by sequential CVD, onto surfaces thatmay be partially nonmetallic initially, it is possible to have thedeposition be selective. For example, we have found that when attemptingto coat a sapphire sample placed on a stainless steel holder withtantalum from TaCl₅ vapor and hydrogen atoms, that the tantalum onlyformed on the stainless steel and not on the sapphire. This appears tooccur because the H radicals are more likely to react with the Al₂O₃surface rather than the adsorbed monolayer of TaCl₅.

A similar phenomena was observed in a recent paper (P. Martensson andJ-O. Carlsson, J. Electrochem. Soc. 145, 2926 (1998)) describing athermal sequential CVD deposition of thin copper films onto platinum,but not onto glass.

A way to prevent this selectivity, when it is not desired, would be todeposit the metal oxide over the entire wafer surface. This initialmonolayer of oxide could then be reduced to the pure metal with hydrogenatoms (see Example 9 above). Subsequent layers could be deposited by thedirect reduction of a suitable precursor (e.g. tantalum from TaCl₅ andH).

The commercial applications of the films deposited by the technique ofthis invention should not be limited to this method of creating thesefilms. In some instances, films grown by known sequential CVDtechniques, without resort to radicals may be adequate depending on theapplication.

While the invention has been illustrated in particular with respect tospecific methods of carrying out the same, it is apparent thatvariations and modifications can be made. It will be apparent from thepreceding that the present invention significantly advances the state ofthe art in the technology of sequential chemical vapor deposition ofthin films, and describes several commercially significant applicationsfor films deposited by the method of this invention. The process of thisinvention is unique in that it allows, for the first time, thedeposition of perfectly conformal and very pure films of any compositionat low temperatures.

1. A process of growing silicon dioxide thin film on a part in a chamber by a process comprising a plurality of deposition cycles, wherein each cycle comprises, in sequence: introducing a first gaseous reactant comprising silicon and oxygen into the chamber, such that the first reactant adsorbs on the part; removing gases from the chamber; exposing the part with the adsorbed first reactant to atomic oxygen, such that the atomic oxygen converts the first reactant on the part to silicon dioxide; and removing gases from the chamber.
 2. The process of claim 1, wherein the first reactant is tetraethoxysilane.
 3. The process of claim 1, wherein the atomic oxygen is generated in a radical generator in communication with the chamber.
 4. The process of claim 1, wherein the atomic oxygen is generated by surrounding the chamber with an RF excitation coil.
 5. The process of claim 1, wherein removing the gases from the chamber comprises flowing a purge gas over the part.
 6. The process of claim 5, wherein the purge gas comprises an inert gas.
 7. The process of claim 5, wherein the purge gas comprises oxygen.
 8. The process of claim 7, wherein exposing the part to atomic oxygen comprises exposing the part to radicals created by a plasma discharge in the purge gas.
 9. The process of claim 1, wherein the first reactant adsorbs on the part in a self-limiting manner.
 10. The process of claim 1, wherein more than one monolayer of silicon dioxide thin film is deposited per deposition cycle.
 11. The process of claim 1, wherein at least some of the first gaseous reactant condenses on the surface of the part. 